Gas turbine engines for aircraft achieve thrust by discharging hot gas through the exhaust nozzle.
A variable area convergent/divergent nozzle is necessary in order to achieve good efficiency for multi-mission applications. The maximum efficiency is obtained with independent control of the exhaust nozzle throat and exit areas. With this, the maximum expansion of the gases and therefore maximum thrust is achieved at all times.
One of the goals of the designers is to increase the aircraft's maneuverability, in those flight conditions in which the aerodynamic control surfaces begin to lose their efficiency. A way of achieving this goal is by vectoring the gas from the axial direction to achieve a thrust component which is up, down or sideways. At present, there are several solutions with two-dimensional convergent/divergent nozzles, such as that disclosed in U.S. Pat. No. 4,763,840. But these nozzles can only orient the gas in one direction (generally pitch) and they are heavier than the axisymmetric convergent/divergent nozzles.
There are different systems of 3-D thrust vectoring convergent/divergent nozzles. All these systems can be classified into three major groups:
Those systems that orient the whole exhaust nozzle upstream of the convergent section. Due to the fact that the alteration of the geometry is done upstream of the throat, perturbations are induced in the turbine. Moreover, a highly complicated sealing device is required. PA1 Those systems that orient the flow at the outlet itself of the exhaust nozzle more downstream of the nozzle. With these systems there is an efficiency reduction and increased weight due to the additional device required. PA1 Those systems that orient the divergent segment of the exhaust nozzle. With these systems the perturbation upstream of the turbine is minimized. The increase in weight is less than in the previous case, due to the fact that the nozzle itself orients the flow without the help of any other additional device.
Within this third group, the most common embodiment includes, for example, as shown in U.S. Pat. No. 5,082,182, a convergent section that consists of a plurality of master petals and a plurality of slave petals in order to provide an adequate sealing. The throat area is governed by the well known mechanism of cams, rollers or other mechanisms, such as perimetric mechanism. The control of the throat area requires a number of linear actuators.
Downstream of this convergent section there is a divergent section consisting of the same plurality of divergent master petals and a plurality of slave petals in order to provide an adequate sealing. The divergent master petals are connected to convergent master petals by universal linkages. The linkages permit the lateral and radial motion of the divergent master petals allowing orientation of the flow.
The divergent master petals are linked to an external ring by load struts. The connection of the strut to the divergent master petal is carried out via a spherical linkage, while the connection to the external ring is made via a cylindrical linkage.
Both outlet area and flow orientation is controlled by the external ring. This external ring requires at least three linear actuators.
With this embodiment, two independent actuator systems are required, internal and external.
Another variable geometry nozzle is disclosed in EP 0557229B1, characterized by the fact that it controls the throat area, outlet area and flow orientation only with a set of linear actuators. That supposes a reduction in cost and a more simple design of the actuator system.
This single system consists of three rings, concentric among themselves and with the axis of the turbine, and a plurality of linear actuators linked by their upstream end to the structure of the turbine.
The rings mentioned above are connected together and to the structure of the turbine via linkage elements and guide devices which allow the joint axial displacement of the three rings in equal magnitude, with respect to the structure of the turbine, as well as a relative rotary movement of the intermediate and external rings between themselves and with respect to the internal ring, thereby allowing the external ring to be inclined in any direction with respect to the center of rotation in the axis of the turbine.
The convergent master petals are connected at a point in their upstream segment to the internal ring by cylindrical linkages, tangential to a theoretical circumference which is concentric to the longitudinal axis of the engine and located in a theoretical plane perpendicular to such longitudinal axis.
The master petals of the divergent section are subdivided transversely into at least two segments that are connected to each other by a cylindrical linkage perpendicular to that of a linkage between the master petal of the convergent and divergent sections. The divergent master petals are connected at a point in their upstream segment to the master convergent petals by cylindrical linkages, parallel to the linkage between the internal ring and the master convergent petals. The downstream segment is connected to the external ring via a load strut that links to that segment by a spherical linkage and to the external ring by a cylindrical linkage, tangential to a theoretical circumference which is concentric to the longitudinal axis of the engine and located in a theoretical plane perpendicular to such longitudinal axis. The invention is related to said load strut.
The external ring is connected to downstream ends of the linear actuators by spherical linkages. The external ring is divided into two half rings that are connected to each other by a cylindrical linkage, perpendicular to the theoretical axis of the engine. The linkage between the half rings mentioned allows a relative rotary movement of the external half rings between themselves achieving the outlet control or allowing the external ring to be inclined in any direction with respect to the center of rotation in the axis of the turbine as a unitary ring, orienting the flow in any direction.
In patent EP 0557229B1 the master petals are distributed in a way that half of them are connected to one of the two external half rings via a load strut, and the other half are connected to the other external half ring.
This distribution limits outlet control due to petal interferences and sealing problems. If two of the master petals are located in the linkage between the two external half ring, the limitation of outlet control is reduced.
This new distribution of the petals needs a different load strut. The new load strut must allow that the divergent master petals are connected to both external half rings at the same time.